The present invention relates to a driving circuit for an electro-luminescent (EL) element and an EL display using the driving circuit. More particularly, the invention relates to a display comprising a combination of a step-up coil, etc. and a semiconductor IC device.
Recently, an EL display using an EL element is used as a back light or for displaying characters and the like in a face of a wristwatch and a display panel of portable information equipment.
Such a conventional EL display will now be described with reference to the accompanying drawings.
FIG. 10 is a circuit diagram of the EL display using a conventional driving circuit for an EL element. In FIG. 10, a reference numeral 51 denotes an EL element used for light emission of a display panel, a reference numeral 52 denotes a DC power supply, a reference numeral 53 denotes a coil for generating a voltage to be applied to the EL element 51 by increasing a voltage of the DC power supply, and a reference numeral 54 denotes a diode for preventing a voltage in the reverse direction from being applied to the coil 53. In an area 100 integrated on a semiconductor substrate, a reference numeral 55 denotes a step-up transistor of an N-channel MOS transistor grounded at its source electrode, a reference numeral 56 denotes an oscillator, which is connected with the gate electrode of the step-up transistor 55 at its output terminal, for determining a switching frequency of the step-up transistor 55, a reference numeral 57 denotes a frequency divider for determining a frequency of an AC signal generated by inverting the polarity of the voltage applied to the EL element 51, and a reference numeral 60 denotes a switching circuit for generating the AC signal by inverting the polarity of the voltage applied to the EL element 51.
The switching circuit 60 includes a first N-channel MOS transistor 61 and a second N-channel MOS transistor 62, which are connected with the cathode of the diode 54 at their drain electrodes and with a first output terminal X or a second output terminal Y at their source electrodes, serving as a switch for inverting the polarity of the voltage applied to the EL element 51; a third N-channel MOS transistor 63 which is grounded at its source electrode and connected with the source electrode of the first N-channel MOS transistor 61 at its drain electrode and with the gate electrode of the second N-channel MOS transistor 62 at its gate electrode; a fourth N-channel MOS transistor 64 which is grounded at its source electrode and connected with the source electrode of the second N-channel MOS transistor 62 at its drain electrode and with the gate electrode of the first N-channel MOS transistor 61 at its gate electrode; and an inverter 65 for inverting the polarities of a signal applied to the gate electrodes of the first and third N-channel MOS transistors 61 and 63 and a signal applied to the gate electrodes of the second and fourth N-channel MOS transistors 62 and 64 mutually to one another.
In FIG. 10, arrows added to symbols of the N-channel MOS transistors 61, etc. correspond to their source electrodes, which is applicable to other drawings described hereinafter.
Now, the operation of the EL display having the aforementioned configuration will be described. As is shown in FIG. 10, the frequency of a gate voltage V56 applied to the gate electrode of the step-up transistor 55 is determined by the oscillator 56, and the variation of the gate voltage V56 with time is shown in FIG. 11. In FIG. 11, T indicates a cycle of the gate voltage V56, and V65 indicates an output voltage of the inverter 65 determined on the basis of the oscillating frequency of the oscillator 56 and the dividing ratio of the frequency divider 57. In this case, the frequency of the output voltage V65 is approximately 400 Hz. At this point, when the dividing ratio of the frequency divider 57 is set at, for example, 1/16, the cycle of the output voltage V65 is 16 T. In a period while t=0 through 8 T (i.e., a period a in FIG. 11), the output voltage V65 is at a low level, and hence, the second and third N-channel MOS transistors 62 and 63 are in an off-state and the first and fourth N-channel MOS transistors 61 and 64 are in an on-state. In such a case, a voltage at the second output terminal Y connected with the EL element 51 is GND, and a voltage at the first output terminal X is increased up to, for example, approximately 50 through 80 V by the coil 53 and the step-up transistor 55.
V51 in FIG. 11 indicates a voltage applied to the EL element 51 on the basis of the voltage at the second output terminal Y. In a period while t=8 T through 16 T (i.e., a period b in FIG. 11), the output voltage V65 of the invertor 65 is at a high level, and hence, the second and third N-channel MOS transistors 62 and 63 are in an on-state and the first and fourth N-channel MOS transistors 61 and 64 are in an off-state. In this case, the voltage V51 applied to the EL element 51 is discharged through the drain-source conductive path of the third N-channel MOS transistor 63. Therefore, contrary to the period a, the voltage at the first output terminal X connected with the EL element 51 is GND and the voltage at the second output terminal Y is increased.
In this manner, the conventional EL display includes the EL driving circuit for increasing the voltage applied to the EL element and inverting the polarity thereof on the basis of the cycle determined by the oscillator 56 and the frequency divider 57.
In the conventional driving circuit for the EL display, however, the polarity of the voltage applied to the EL element is inverted in a predetermined cycle (that is, 8 T in the aforementioned case). This leads to the following three problems:
First, in the case where a plurality of EL elements are included in the display, voltages applied to the respective EL elements are varied in accordance with fluctuation in the capacitance among the EL elements. This variation in the voltages disadvantageously results in brightness irregularity in the entire display. Such conventional brightness irregularity will be described with reference to FIG. 12, which shows the conventional variation in the voltage applied to the EL element in accordance with the fluctuation in the capacitance of the EL element. In FIG. 12, a solid line indicates a voltage applied to the EL element when its capacitance is relatively small, and a broken line indicates a voltage applied to the EL element when its capacitance is relatively large. As is shown in FIG. 12, in the conventional driving circuit for the display including plural EL elements, the voltages applied to the respective EL elements are varied in a range of approximately .+-.20% in accordance with the fluctuation in the capacitance of the EL elements. Accordingly, the brightness of the respective EL elements is not uniform, which causes the brightness irregularity in the entire display.
Second, in the case where the EL element is disconnected for a change or some other reason, a capacitance much larger than the capacitance of the transistor is lost. Since a capacitance is in inverse proportion to a voltage, the loss of the capacitance increases the drain voltage of the transistor to exceed the breakdown voltage of the transistor. This can disadvantageously damage the transistor. Specifically, when the EL element is disconnected for a change or some other reason, merely the capacitances of the drain electrodes of the first through fourth N-channel MOS transistors 61 through 64 of the switching circuit 60 remain as the load for the driving circuit of FIG. 10. These capacitances are several pF through several tens pF, which is far smaller than the capacitance of the EL element 51, i.e., 1000 pF or more. Accordingly, owing to the voltage increased by the coil 53, the drain voltages of the N-channel MOS transistors 61 through 64 are increased to exceed their drain breakdown voltages.
Thirdly, as is shown in FIG. 12, the waveform of the voltage applied to the EL element is steep at the fall from the increased voltage to 0 V in switching the polarity. Accordingly, since the frequency for switching the polarity is approximately 400 Hz, this steep fall in the waveform of the voltage results in applying oscillation to the EL element. This can disadvantageously cause a noise. Particularly in portable communication equipment such as a portable telephone and other electronic equipment in which a noise caused in use leads to nonconformity, the oscillation noise of the EL element is desired to be decreased. Therefore, it is indispensably necessary to improve the waveform of the voltage applied to the EL element.